Heat the oven to 400°F. Combine the chickpeas and 2 tablespoons of the oil in a large roasting pan and bake, shaking the pan once or twice, until they’re crisp and browned, 15 to 20 minutes. Sprinkle with salt and pepper, toss, and remove the chickpeas from the pan with a slotted spoon.
Add the remaining 1 tablespoon oil to the pan along with the lamb. Break it up a bit and roast until it is no longer pink and has cooked dry, 5 to 10 minutes. Stir, breaking it up more, and continue baking until it begins to crisp, another 5 minutes or so.
Stir the onion, green beans, garlic, orange zest, cumin, and cayenne into the lamb; sprinkle with salt and pepper. Return the roasting pan to the oven and continue roasting until the vegetables are tender and browned, the lamb is very crisp, and the spices are fragrant, 10 to 15 minutes. Remove the pan from the oven, stir in the chickpeas, and transfer everything to a serving dish.
Put the roasting pan on the stovetop over medium heat. Add the orange juice and cook, stirring and scraping up any brown bits on the bottom of the pan, until warm and slightly thickened, 5 to 10 minutes. Drizzle the orange reduction over the lamb and chickpeas and taste and adjust the seasoning. Garnish with the parsley and serve.